A Novel Stochastic Mechanics Framework for Reliability Analysis of Offshore Mechatronic Subsea Systems Under Deep Water Pressure and Corrosion Loading in Nigerian Waters

Subsea mechatronic systems including remotely operated vehicles, subsea control modules, and hydraulic flying leads deployed in the Nigerian deepwater oil province at depths exceeding 1,000 metres are subjected to combined mechanical, thermal, and electrochemical loading from hydrostatic pressure, low-temperature deep sea water, and hydrogen-induced stress corrosion that interact through complex non-linear damage mechanisms underrepresented in current offshore reliability standards. This dissertation develops a novel stochastic mechanics framework for the reliability analysis of offshore mechatronic subsea system components, integrating probabilistic fracture mechanics, coupled diffusion-damage constitutive modelling for hydrogen-assisted cracking, and extreme value statistics for deep water environmental loading characterization. A key theoretical innovation is a stochastic hydrogen diffusion and trapping model for high-strength steel subsea structural components that explicitly represents the probabilistic spatial distribution of hydrogen concentration driven by cathodic protection potential variability, yielding failure probability estimates that are formally connected to in-situ cathodic protection system performance metrics. The framework is validated against a dataset of 31 documented subsea hardware failures compiled from Nigerian deepwater operational records provided under non-disclosure agreement by three international oil companies. Bayesian model updating is employed to calibrate stochastic model parameters to the Nigerian deepwater failure record, demonstrating substantially improved predictive accuracy over uncalibrated engineering standards approaches. An optimal inspection and maintenance policy for subsea mechatronic systems, derived from the reliability framework using dynamic programming, reduces expected 10-year lifecycle inspection cost by 33 percent relative to interval-based policies. Keywords: reliability analysis, subsea mechatronic, stochastic mechanics, hydrogen embrittlement, Nigerian deepwater